This invention relates to the field of well logging and, more particularly, to improvements in the automatic monitoring of well logging information for quality control and other purposes. The invention is applicable to wireline well logging and also to logging while drilling.
The monitoring of the general quality of borehole logging data is a routine procedure for field personnel that are responsible for running logs at a wellsite. Typically, the field engineer will observe logging data as it is being collected to determine if it is within an expected range, and if its behavior appears consitent with readings that are normally obtained when running the type of logging device that is being used. If and when an abnormal or anomalous condition is observed, and if the observed condition is recognized as being indicative of an identifiable problem, the field engineer may be able to take appropriate corrective action. For example, he may make adjustment of controls, replace a defective device, and/or rerun a section of logging data while the opportunity still exists for doing so.
For a number of reasons, there are limitations on the present ability of field engineers to recognize anomalies and respond to them during or just after the logging process. To understand these limitations, and their consequences, consider the following:
In various applications, a number of logging devices are run simultaneously, and the field engineer has to divide his attention between various types of arriving logging data.
Most field engineers are well trained, but experience levels vary considerably. Even well trained logging personnel cannot be expected to know every nuance of the many available logging devices. Therefore, there are necessarily human limitations in the ability to recognize anomalies, much less diagnose them--particularly during the time frame of the logging operation when many things are happening at once.
When a logging run has been completed, there is generally little time available to make decisions regarding the adequacy of the logging information which has been obtained. The enormous expenses associated with drilling and completion dictate that the next phase of work start almost immediately, and the opportunity to re-run logs under the same or similar circumstances may be irretrievably lost. In some situations, when the logging truck leaves the well site, the next step is the setting of casing. Many logging tools work only in open-hole and cannot measure formations through casing. If log quality problems are found after the casing has been set, it will be too late to re-run faulty logs.
A recently devised approach uses computer-generated "quality curves", which process selected logging signals and display readings of the quality of logging information. The provision for simple checks of log value extremes also exists. However, existing computer-generated log quality approaches suffer one or more of the following shortcomings: lack of flexibility in the type of applicable logging devices and/or conditions monitored; increase in the amount of information that the engineer must cope with; inability to provide information concerning the cause of a sensed anomaly; inability to assist in diagnosing the cause of the problem; inability to provide confirmation that a problem exists; and inability to operate and/or produce useful assistance "on the fly", i.e., in real time.
It is among the objects to the present invention to provide a log monitoring system and method which is responsive to these shortcomings of the prior art.
Another prior art practise which is well established, but could stand improvement, pertians to the use of logs from offset wells (and/or other sources of local geological knowledge) for comparison with logs being run in a present well to check that the log values are within expected ranges. Typically, a number of geological markers are identified, and it is attempted to match logging responses against these markers. However, existing techniques for using markers suffer some of the same types of shortcomings that are mentioned above; particularly, inability to provide, during the logging run, marker identification and accompanying real-time log quality evaluation.
It is among the further objects of the invention to provide improvement in techniques and apparatus for use of local geology information in efficiently and accurately locating geological markers in a well being logged, in evaluating quality of logs which should respond to the marker, and in doing so during the logging operation itself.